Question: 9 binders cost $12.51. Which equation would help determine the cost of 13 binders?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 9 binders. We want to know the cost of 13 binders. We can write the numbers of binders as a proportion: $\dfrac{9}{13}$ We know 9 binders costs $12.51. We can let $x$ represent the unknown cost of 13 binders. The proportion of these costs can be expressed as: $\dfrac{\$12.51}{x}$ The cost changes along with the number of binders purchased, and so the two proportions are equivalent. $\dfrac{9}{13} = \dfrac{\$12.51}{x}$